Effect of foot immersion and neck cooling on renal, intestinal, immune and inflammatory markers in older adults exposed to extreme heat.

Older adults with reduced thermoregulatory capabilities are increasingly at risk of heat-related pathophysiological outcomes (e.g., acute kidney injury, heatstroke) due to increasingly frequent, prolonged and intense heatwaves. Foot immersion and neck cooling have been proposed as practical, non-electrical cooling strategies for protecting older adults during heatwaves, though evidence supporting their efficacy is limited. This study evaluated the effect of foot immersion with or without neck cooling on systemic proteins associated with acute kidney injury (NGAL, KIM-1), intestinal enterocyte damage (IFABP), immune activation (sCD14) and systemic inflammation (IL-6, TNF-α, CRP) in older adults. Seventeen participants (nine females; median [IQR] age 72 [69-74] years) completed three randomized 6-h passive heat exposures (38°C, 35% relative humidity) with no-cooling, foot immersion in 20°C water, or foot immersion with neck cooling via wet towels. Thermal and cardiovascular strain were measured throughout exposures, with venous blood samples collected pre- and post-exposure. Body core temperature increased by ∼1.1°C (P < 0.001) with no changes in any measured systemic proteins (all P > 0.05) across conditions. Foot immersion with or without neck cooling modestly reduced heart rate, mean skin temperature, whole-body sweat rate and fluid consumption (P < 0.05), but had no effect on body core temperature or systemic protein concentrations (all P > 0.05) relative to no-cooling. These findings do not support the efficacy of these interventions for mitigating hyperthermia in older adults during heatwaves. Further research is warranted to evaluate their efficacy for protecting against heat-related acute kidney injury, intestinal enterocyte damage, immune activation, or systemic inflammation under more severe exposure conditions.